How to make my function generic

    let csvList startDelim endDelim lst memF = 
        let listIter (listStr: string) item = 
            if listStr.Length > 0 then 
                listStr + "," + (memF item)
            else 
                memF item                
        startDelim + (List.fold listIter "" lst) + endDelim

    let listIntoJsonArray = csvList "[" "]"
    let listIntoJsonObject = csvList "{" "}"

    let intConverter (item:int) : string =
        item.ToString()

    let objectConverter (item:SomeObject) : string =
        item.value.ToString()

    let objects = [{vaue: 12.3}; {vaule: 33.2}]
    let ints = [1;2;4]

    let o = listIntoJsonObject objects objectConverter
    let i = listIntoJsonObject ints intConverter 

I can't seem to find the magic sauce to make the csvList or its partially applied helpers listIntoJsonArray or listIntoJsonObject generic.

Thanks for the help.

Your listIntoJsonArray and listIntoJsonObject are values, not functions, so you met value restriction monster. You can either convert them to functions by adding explicit argument or to type functions.

// function case

let csvList startDelim endDelim lst memF = 
    let listIter (listStr: string) item = 
        if listStr.Length > 0 then 
            listStr + "," + (memF item)
        else 
            memF item                
    startDelim + (List.fold listIter "" lst) + endDelim

let listIntoJsonObject x = csvList "{" "}" x

let ints = [1;2;4]

let i = listIntoJsonObject [1;2;4] string 
let y = listIntoJsonObject ["123"] id


// type function case

let csvList<'T> startDelim endDelim (lst : 'T list) memF = 
    let listIter (listStr: string) item = 
        if listStr.Length > 0 then 
            listStr + "," + (memF item)
        else 
            memF item                
    startDelim + (List.fold listIter "" lst) + endDelim

[<GeneralizableValue>]
let listIntoJsonObject<'T> = csvList<'T> "{" "}"

let ints = [1;2;4]

let i = listIntoJsonObject [1;2;4] string 
let y = listIntoJsonObject ["123"] id